Let a=(pi)/(7), then (a) show that sin...

Let `a=(pi)/(7)`, then
(a) show that `sin^(2)3a-sin^(2)a=sin2asin3a`
(b) show that `co sec a=co sec 2a+co sec4a`.
(c) Prove that `cos a` is a root of the equation `8x^(3)+4x^(2)-4x+1=0`.

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